1. Field of the Invention
This invention relates generally to a battery feed circuit for telecommunications and more particularly to a low cost battery feed using resistors.
2. Description of the Related Art
A battery feed circuit supplies a DC feed circuit to telecommunications equipment through a 2-wire transmission path that also typically conducts a bi-directional voice or data signal. The feed current is balanced, i.e., each transmission wire conducts a current that is equal in magnitude but opposite in direction to the other.
A variety of battery feed circuits have been developed. In general, these circuits can be grouped into one of two classes depending on their battery feed profile, that is the relationship between the DC feed circuit and the voltage across the 2-wire transmission path. The first class of battery feed circuits produces a linear battery feed profile, e.g. U.S. Pat. No. 4,004,109 to F. S. Boxall, having a low common mode impedance to longitudinal signals. The second class of battery, feed circuits, such as those disclosed in publication "A Floating Low-Power Subscriber Line Interface" by L. Freimanis and D. P. Smith, ISSCC Digest, 1980, pages 180, 181, provide a non-linear battery feed profile which limits feed current on short 2-wire transmission paths. The noise induced by longitudinal signals is minimized by providing a high common mode impedance through the use of isolation devices, e.g. transformers or opto-isolators. These devices, however, tend to be bulky and expensive. One prior art attempt to deal with this problem is in U.S. Pat. No. 4,476,350 to Aull et al. by the use of hi-directional current sources.